Hollywood actor Martin Sheen has declared himself proud of his uncle's IRA past.

The star said he was also relieved to discover that his mother's brother, Michael Fieland, from Co Tipperary, had no part in the assassination of Irish revolutionary leader Michael Collins.

Sheen found out about his close family links to Ireland's War of Independence while taking part in the US version of the hit genealogy television series Who Do You Think You Are?. The 71-year-old visited Dublin's Kilmainham Gaol and spent time in the cell where it is believed his uncle was incarcerated.

"I'm enormously proud of him," he said. "I would like to hope that if I had been here in Ireland at the time, I would have followed him. And I would have been as committed as he was."

Best known for his roles in Apocalypse Now, Wall Street and the television series West Wing, he described his uncle as an Irish volunteer. The actor said Fieland went on to fight against the Free State side, who supported the Anglo-Irish treaty, during the resulting Civil War in the early 1920s.

Sheen was concerned about what the TV researchers would turn up. "When I was in Ireland and discovering the involvement of my uncle in the Rising and the Civil War, because he took an opposing side to (Eamon) de Valera, I was afraid he might have been in on the plan to assassinate Mick Collins," he said.

"But as it turned out he was in prison when Mick Collins was assassinated and I was deeply relieved."

One of 10 children, Sheen, whose real name is Ramon Estevez, was born to a Spanish father and Irish mother, Mary-Anne Fieland, from Borrisokane in Co Tipperary. She emigrated to the US during the War of Independence, and he believes she was sent away to protect her from the intense hostilities.

"And she was meant to come back when the fighting stopped and the Republic was established in 1923," he said. "And so it was a very, very satisfying moment for me to know that she, too, was involved."

Sheen fulfilled a life dream by returning to his mother's homeland to study in 2006 when he took a place as a mature student at the National University of Ireland in Galway.